1828, intransitive, "find fault constantly;" by 1840, intransitive, "annoy by continued scolding, pester with petty complaints," originally a dialectal word meaning "to gnaw" (1825, Halliwell), probably ultimately from a Scandinavian source (compare Old Norse gnaga "to complain," literally "to bite, gnaw," dialectal Swedish and Norwegian nagga "to gnaw"), from Proto-Germanic *gnagan, related to Old English gnagan "to gnaw" (see gnaw). As a noun, 1894, "act of nagging;" by 1925, "person who nags." Related: Nagged; nagger; nagging.
"old horse," c. 1400, nagge "small riding horse, pony," a word of unknown origin, perhaps related to Dutch negge, neg (but these are more recent than the English word), perhaps related in either case to imitative neigh. The term of abuse "a worthless person," often of a woman, is a transferred sense, first recorded 1590s. For "one who annoys by scolding" (by 1925) see nag (v.).